1. Field of the Invention
The present invention relates to a process for preparing a toner resin, the toner resin prepared thereby and a toner composition containing particles of the toner resin.
2. Description of the Related Art
Styrene based copolymers, particularly styrene-butadiene copolymers, have long been known to be useful as toner resins that can be used in toner compositions and developer compositions in electrostatic imaging systems. For instance, U.S. Pat. No. 4,469,770 discloses styrene-butadiene plasticizer toner composition blends comprised of from about 40 to about 94.5 weight percent of a blend of styrene-butadiene copolymer resin particles and a plasticizer composition which functions as a surfactant prior to acid coagulation. The copolymer contains from about 85 to about 93 weight percent of styrene and from about 7 to about 15 weight percent of butadiene and has a weight average molecular weight of from about 45,000 to about 155,000 and a number average molecular weight of from about 7,000 to about 25,000. In column 1 of the patent, reference is made to other patents which describe styrene-butadiene copolymers used in toner resins, namely U.S. Pat. Nos. 3,326,848, 3,960,737, and 3,766,072. Other references to styrene-butadiene copolymers as toner resins are in U.S. Pat. No. 4,,148,937 and Japanese Kokai Publication No. 53-25654.
The art has generally recognized that the molecular weight of the toner resin is an important consideration. In U.S. Pat. No. 4,558,108, a vapor phase-aqueous phase process employing multistage heating is used to prepare styrene-butadiene toner resin particles having a weight average between about 10,000 and 400,000 and a glass transition temperature of between about 50.degree. and 130.degree. C. The toner particles are stated as having a polydispersity up to about 9 which is contrasted with a single stage heating process that has a polydispersity of between about 2 and about 5. Other patents discussing the molecular weight of the toner resin include U.S. Pat. Nos. 4,473,628, 4,557,991, 4,565,766, 4,652,511, and 4,702,986 and Japanese Kokai Publication No. 57-5052.
U.S. Pat. No. 4,564,573 describes an electrostatic image developing toner comprised of a binder resin which contains at least 60 percent by weight of styrene-butadiene copolymer containing a component A having a molecular weight of at least 100,000 and a component B having a molecular weight of at least 500,000. U.S. Pat. No. 4,473,628 further describes a toner resin which is prepared from two different types of styrene-butadiene copolymers.
In preparing polymers which can be used as toner resins, the art has utilized a variety of materials, such as initiators and surfactants, which are used depending on the process selected. One such material is a chain transfer agent and, in this utility, the art has used mercaptans, particularly dodecyl mercaptan. For example, aforementioned U.S. Pat. Nos. 4,473,628 and 4,564,573 disclose the presence of t-dodecyl mercaptan in the polymerization initiation system for a copolymer which is predominantly prepared from styrene and butadiene.
Despite the extensive art which describes styrene-butadiene toner resins, the search has continued for efficient processes which can prepare toner resins having characteristics which make them particularly suitable in toner and developer compositions.